NEW DELHI (Reuters) - India's only private weather agency on Thursday warned against "irrational exuberance" after a state forecast flagged heavier-than-expected monsoon rains and the stocks of companies dependent on the farm economy shot ahead a second straight day.

The Indian Meteorological Department (IMD) told Reuters late on Tuesday that the June-September monsoon season - critical for the half of the country's farmland that is not irrigated - was likely to bring more rain than its previous forecast as concerns over the El Nino weather condition had eased.

Indian shares have been on a record-breaking spree since, but privately held Skymet said it was sticking to its forecast of below-normal monsoon rains.

"I am cautiously optimistic but I need more data - which will come between a week and 10 days - to take a better call," Skymet CEO Jatin Singh told Reuters.

"The El Nino forecast has become volatile, (and) next week it might go up again. We will have to see it week by week."

El Nino, a warming of ocean surface temperatures in the eastern and central Pacific, typically occurs every few years and is linked to crop damage, fires and flash floods.

Australia's Bureau of Meteorology said this week some international climate models have reduced the likelihood of El Nino this year compared to indications last month. Five of eight such models, however, still indicate the tropical Pacific Ocean may exceed El Nino thresholds during the second half of 2017. (http://bit.ly/1k3blIG)

Kyle Tapley, an agriculture meteorologist with U.S.-based MDA Weather Services, said he expected a slightly below-normal monsoon this year in India but nothing "catastrophic".

"During the monsoon season in India we will be probably transitioning into El Nino, so I don't think there will be huge impact but we are showing a little bit of drier influence," Tapley told Reuters.

IMD, which on April 18 forecast this year's monsoon rains at 96 percent of the 50-year average of 89 cm, cautioned though that weather patterns are dynamic.

India defines average, or normal, rainfall as between 96 percent and 104 percent of the 50-year average.

The federal farm ministry has already written to various states to review their preparedness to manage impact on farmers in case of below-normal rains, it said in a statement on Monday.

Government data shows that only once in the last four years have actual rains been higher than IMD's official forecasts.